This invention relates to autocollimators and, in particular, to autocollimators adapted to make measurements over a wide angular range. Autocollimators are known telescope-like optical instruments for measuring small angular displacements of a test object from a plane in a line of sight of the instrument. A beam of light is projected onto a reflective surface of the test object and reflected back to a light detector. Changes in the path of the reflected light beam back to the reflector, resulting from angular displacements of the test object, are detected and measured. Such autocollimators are the subject of U.S. Pat. No. 3,554,653, 3,542,478; 3,977,789; 3,470,377; 4,650,298; 4,774,405; 4,890,917; 4,909,625; and 4,975,565; the subject matter of which is incorporated herein by reference.
In general, autocollimators are "static" measuring devices, i.e., they are precision devices used for measuring the results or progress of scientific experiments and manufacturing processes, e.g., in checking the flatness of machine beds and surface plates. A good example of such an autocollimator, and one which has particular relevance to the present invention, is disclosed in U.S. Pat. No. 3,554,653. This patent (Zielke et al.) discloses a pair of modulated light sources for directing two slightly spaced apart beams of light towards an external reflector mounted on an object being measured. In the null position, the light reflected from neither beam enters a light receiving aperture of a light sensor. However, upon angular displacement of the external reflector, one or the other reflected beams of light enters the aperture and generates a signal indicating that a displacememt has occurred and the direction of the displacement. For actual measurement of the angular displacement, the light beams are caused to pass through a movable optical wedge, the position of which causes a variable displacement of the paths of the reflected beams. In use, the apparatus is first arranged to provide a zero or null output signal, a measurement is then made resulting in an output signal, and the optical wedge is then moved in a direction for re-establishing the null condition. The amount of movement of the wedge is a direct measurement of the angular displacement of the test object. The mechanical system disclosed in the patent is obviously quite slow in operation and has limited utility, even if mechanized for faster operation, in systems requiring substantially instantaneous feedback information.
One application requiring such substantially instantaneous feedback, and in which the present invention has particular utility, is in the aiming of the armaments of moving vehicles, in particular, the aiming of the main gun or cannon of an armoured vehicle, e.g., a battle tank as shown in FIG. 6.
As known, such guns are optically aimed, e.g., using laser beams, and for accurate firing, it is essential that the gun barrel be precisely aligned with the line of sight of the optical aiming device. A problem, however, is that when the vehicle is moving, the gun barrel, (see FIG. 6) which can be quite long, e.g., 15 feet, experiences small degrees of bending even with highly effective gyroscopic stabilizing systems. Even small barrel deflections, particularly with the longest length barrels, provide a significant deviation between the true aim of the barrel and the line of sight of the aiming mechanism.
A proposed solution is to measure the degree of deflection of the gun muzzle with respect to its trunion and to correspondingly adjust the aim of the gun taking into account the amount of deflection. In a dynamic situation, however, i.e, in a moving vehicle where the degree of deflection is constantly changing, the adjustment must be extremely rapid and the measurements of the angular displacement of the muzzle must be correspondingly rapid. The proposed solution utilizes an autocollimator, but it is clear that the autocollimator disclosed in the aforediscussed Zielke et al. patent operates far too slowly for this operation.
Another problem in the moving vehicle application is that at high speeds and over rough terrain the range of angular displacement of the gun muzzle can be quite large in comparison with the range of displacements measurable by typical autocollimators. Thus, the autocollimator used must be capable of measuring unusually large deviations. In the Zielke et al. arrangement, the range of measurements possible is determined by the range of movements of the optical wedge. While a large range of measurements is thus possible, as above-noted, the mechanical system of Zielke at al. suffers from slow speed.
Further, even in the Zielke et al. arrangement, an ultimate limitation on the maximum range of possible measurements is that with excessively large angular deviations of the test object, the reflected light follows a path so divergent from the outgoing beam of light as to entirely miss, i.e., pass to one side of, the optical aperture of the light receiving mechanism. One solution is simply to increase the size of the optical aperture. Even this solution has its practical limits and, in any event, is generally undesirable as increasing the size, increases the cost and, particularly in a military operation, the vulnerability of the autocollimator system.
It turns out that, while not recognized by Zielke et al., and not discussed in their patent, the use of two beams of light is an effective means for increasing the range of measurements possible with an autocollimator, and is the means employed in the present invention for achieving an autocollimator measuring system having a wide measuring range relative to the size of the optical aperture of the system.